1. Field of the Invention
The present invention relates to a transmission device including a transmission member and a rotation shaft fitted into a mounting hole, which is formed in the transmission member, by press-fitting, a photosensitive drum fitted to the rotation shaft of the transmission device, a process cartridge including the photosensitive drum, and an image-forming apparatus.
2. Description of the Related Art
Transmission devices having transmission members, each including a gear, a pulley, or a roller, and rotation shafts, each fitted into a mounting hole formed in the transmission member, have been employed in various technical fields. In image-forming apparatuses, for example, a transmission device, in which a transmission member is rotated and the rotation is transmitted to a photosensitive drum via a rotation shaft, is disclosed in Japanese Unexamined Patent Application Publication No. 7-239596 (P7–8, FIG. 6).
In such conventional transmission devices, the outer diameter of the rotation shaft is set to be smaller than the inner diameter of the mounting hole of the transmission member so that the rotation shaft is loosely fitted into the mounting hole during the assembling of the transmission device. Then, the transmission member has been generally fixed to the rotation shaft with a screw so as to enable these members to integrally rotate. By such a structure, when the mounting hole of the transmission member is fitted to the rotation shaft, a large force is not necessary for easily fitting both the members with each other.
However, if the rotation shaft is loosely fitted into the mounting hole of the transmission member, a small clearance is produced between both the members after they are fitted to each other. In this state, if the transmission member is fixed to the rotation shaft with a screw, the transmission member may be assembled eccentrically to the rotation shaft. The rotation transmission by such a transmission device having eccentricity produces an uneven speed of the rotation, so that various problems arise therefrom. For example, when the photosensitive drum is fixed to the rotation shaft while the transmission member including a gear is fixed to the rotation shaft so that the gear is rotated so as to transmit the rotation to the photosensitive drum, as the example described above, if the gear is eccentric to the axial line of the rotating shaft, even when the gear is rotated so as to maintain the angular speed constant, an uneven rotational speed is produced in the gear.
Such an uneven speed is transmitted to the photosensitive drum via the rotation shaft so as to produce inconsistencies in density of toner images formed on the surface of the photosensitive drum. In a color image-forming apparatus, in which toner images on a plurality of photosensitive drums are transferred onto a recording medium directly or via a transcriptive intermediate, because of the uneven rotational speed of the photosensitive drums, color misalignment and changes in a hue are generated in color images transferred on the recording medium, deteriorating the image quality.
Then, the rotation shaft seems to be fitted into the mounting hole formed in the transfer member by press-fitting. In such a manner, when the rotation shaft is completed to fit the mounting hole, a rattly clearance is not produced between both members, substantially eliminating the eccentricity of the transfer member due to the rattly clearance. The above-mentioned defects, therefore, can be prevented from being generated.
However, when the rotation shaft is fitted into the mounting hole of the transmission member by press-fitting, a large force is required for the fitting. In the case where the transmission member is a resin molded product, the inner diameter of the mounting hole may largely change in size up to a range of 20 μm, for example, because of variations during molding and the ambient temperature. Thus, when the inner diameter of the mounting hole is varied in a decreasing direction, a very large press-fitting force is necessary for fitting the rotation shaft into the mounting hole by the press-fitting, greatly deteriorating the assembly efficiency.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Patent Application Publication No. 7-160172, No. 7-243511, No. 2002-227866, and No. 2003-122188.